igneous

Descriptive of rocks or minerals that have solidified from molten or partly
molten material called magma. Igneous (Latin,
ignis = a fire) rock is extrusive
if it formed on the surface of the Earth or other planetary body, and intrusive
if it solidified deep underground. All igneous rocks are crystalline: if
they cooled slowly from magma their crystals are large; if they cool fast
their crystals are small. In addition to grain size, the other important
differences in igneous rocks are related to the chemical content of the
minerals in the rock (see section on classification below)..

Magma from a single chamber may reach the Earth's
surface as volcanic lava or it may cool inside the crust in a variety
of different shaped intrusions. Over a very long period the magma
in the chamber may itself cool into rock.

Extrusive rock

Igneous rocks that cooled on the surface are termed extrusive. The magma
from which they are formed may vary in composition and properties. Viscous
magma forms massive steep-side mountains
– the familiar cones of many volcanoes.
Pockets of trapped gas eventually escape causing violent explosions. Fragments
of half-cooled rock are then thrown into the air. Sometimes the lumps are
quite large – 20 to 30 cm in length. These are volcanic
bombs. As they descend, they twist and turn, and the soft rock becomes
spindle-shaped. Smaller particles are called lapilli,
meaning "little stones," while the finest dust forms volcanic
ash. The molten rock itself flows as a lava
and solidifies, giving off gases. Small cavities or vesicles may, as a result,
be formed inside this rocky giving a frothy appearance. Pumice
stone is a well known example of this phenomenon.

If the magma is more fluid, instead of forming steep volcanoes, it flows
into thin sheets over very large areas. The islands of Hawaii are made up
of such sheets of basalt, a dark, easily-flowing
lava.

As a rule all extrusive rocks are predominantly glassy since they cool too
rapidly for crystals to form.

Intrusive rock

Some magma never reaches the surface but cools within the Earth. The igneous
rocks formed are then termed intrusive. If the magma was injected into sedimentary
layers along bedding planes, the igneous sheet formed is called a sill
and is said to be conformable with the surrounding rock.
Sometimes, the magma forces its way across strata at angles to the bedding
planes. These structures are called dikes and
are said to be unconformable.

It may happen that the magma is forced through a comparatively small aperture
in the layers and reaches an area where it is actually able to lift up the
layers of overhead rock. A bun-shaped intrusion or laccolith
is then formed. Alternatively the magma may cause strata beneath it to sag,
in which case a basin-shaped intrusion or lopolith develops.
Laccoliths and lopoliths are often very rich in valuable minerals. A large
lopolith at Sudbury, Ontario, produces nearly three-quarters of the world's
nickel.

Dikes, sills, and lava flows come from huge chambers of magma that may be
several kilometers below the surface. Usually the magma cools at an intermediate
rate producing an intermediate structure, either a glassy base containing
a few large crystals or a ground mass of very fine crystals. Such rock is
described as hypabyssal. In some areas, the igneous activity
finished many millions of years ago, so that even the magma chambers themselves
have solidified. Rocks cooled at such great depths below the surface are
called plutonic after the Roman god of the underworld,
Pluto. The time taken for solidification to be completed may be millions
of years. Crystals large enough to be seen with the naked eye are able to
form. Examples of such crystalline rocks are granite,
usually light-colored (pink or gray), and the dark, heavy gabbros.

Erosion may lay bare these old magma reservoirs. When of great size they
are called batholiths. Sometimes the roof
of the batholith extends upward into many domes. One of the largest batholiths,
in the United States, is over 1,500 km long and 240 km wide.

Classification of igneous rocks

The elements silicon and oxygen
readily combine to form silicon dioxide or silica
(SiO2). This is a very important mineral. In a molten state it
behaves as an acid, combining with any metallic oxides present to form a
very common and important group of minerals – the silicates.

The quantity of silica present in the original magma is used in classifying
the resulting igneous rock. If a lot of silica is present, it will combine
with all the metal oxides present, and some free silica will remain. Rocks
of this type are said to be acidic. If only a small quantity
of silica remains in the uncombined state, the rocks are described as intermediate.
The so-called basic rocks have even less silica and none
of it remains in the free state. Finally, there are sometimes found heavy,
dark, ultra-basic rocks, so poor in silica that none of
the metal oxides is completely converted to silicates.

All four types of rock may cool either slowly, deep in the crust (plutonic)
rapidly on the Earth's surface (volcanic) or at an intermediate
speed near the surface (hypabyssal).